1. Field of the Invention
The present invention generally relates to a channel estimation apparatus, and more particular, to a channel estimation apparatus with the optimal search applicable to a coherent orthogonal frequency division multiplex system (coherent (OFDM system).
2. Description of Related Art
In recent years, because of the advantages an OFDM system has, such as the high-speed transmission, the high usage efficiency of bandwidth and the good robustness from channel delay and inter-symbol interference, the OFDM system has been broadly applied in digital video broadcasting-terrestrial system (DVB-T system), wireless local area network communication (WLAN communication) and digital audio broadcasting system (DAB system). According to the adopted modulation approach, an OFDM system is categorized into a coherent OFDM system and a non-coherent OFDM system. In terms of the coherent OFDM system, to have a reliable coherent detection capability and to compensate the damage caused by a channel variation, the receiver of the coherent OFDM system must have an accurate channel estimation capability so as to correctly judge the symbols in the received signal.
A coherent OFDM system usually adopts so-called pilot data to assist in conducting a channel estimation. For example, the US Published Patent Application No. 20030138060 discloses a COFDM demodulator with an optimal FFT analysis window positioning. However, the disclosed demodulator does not take account of the effect of the subcarrier channel frequency responses (subcarrier CFRs) on deciding the shift information during a channel estimation process carried by the demodulator. Therefore, the shift information which the demodulator utilises is not the optimal one according to the maximum likelihood estimation (MLE). Moreover, the disclosed demodulator requires huge amounts of the computations for generating the better shift information. In other words, the disclosed demodulator not only is unable to realize the optimal channel estimation, but also fails to effectively advance the operation speed of the system.
In addition, the European Published Patent Application No. EP1580951 discloses a scheme where the delay spread in a channel impulse response is used to assist in estimating the shift information required by a frequency interpolation filter, but the patent application does not explain in detail how to obtain the delay spread in a channel impulse response.